Vegetable and animal fats and oils often contain considerable proportions of free fatty acids. According to the source of the fatty raw material, the content of free fatty acids may be between 0 and 100% by weight. In production processes for biodiesel by transesterifying triglycerides, this proportion of free fatty acids cannot be reacted with methanol to give the corresponding fatty acid methyl esters and leads to yield losses, or to the effect that raw materials with a high content of free fatty acids are unsuitable for biodiesel production.
A pretreatment of the fats is therefore frequently necessary, in which the content of free fatty acids is reduced and the free fatty acids are converted by esterification with alcohol to the fatty acid alkyl ester target product.
The literature discloses deacidification processes for fats and oils, for example by removal of free fatty acids with the aid of a steam distillation (Ullmann's Encyclopedia of Industrial Chemistry, Electronic Edition, Topic “Fats and Fatty Oils”, p. 30). Such a removal allows the acid number of the fats to be reduced to values below 0.2, such that the resulting fats or oils can be converted in a transesterification process.
In this connection, the acid number specifies the mass of potassium hydroxide in mg which is required to neutralize 1 g of the sample to be analysed (DIN 53402, newest version DIN EN ISO 2114).
The literature discloses the esterification of free fatty acids with methanol with the aid of a homogeneous acidic catalyst, e.g. p-toluenesulphonic acid or H2SO4 (Mittelbach, Remschmidt, Biodiesel—The comprehensive handbook 3rd ed., 2006, p. 60). However, this process requires a relatively difficult catalyst removal by neutralization and washing with alcohol or water, which does not allow the catalyst used to be recovered and gives rise to considerable amounts of wastewater.
EP 0192035 and DE 196 00 025 describe a process for deacidifying fats or oils, in which acidic solid ion exchange resins are used as catalysts, and whose removal from the reaction mixture is followed by removal of the water by-product. In this case, the esterification of the free fatty acids is carried out without preceding removal of the free fatty acids in the overall mixture of fatty acid and oil. Since the free fatty acids in most cases make up a comparatively low proportion of the overall amount of fat and oil (usually 2-20 percent by weight, in rare cases even more than 30% by weight), this process leads to an unfavourable space-time yield both in the esterification and in a subsequent transesterification, but more particularly with regard to the esterification of the free fatty acids, since an additional mass flow of inert components is also processed in the reaction.
DE 196 00 025 discloses, inter alia, that it may be advantageous to add additional free fatty acids to the pre-esterification. Under some circumstances, this can serve to reduce the relative proportion by mass of inert material in the starting material of the process. However, no fundamental solution to the technical problem of achieving higher conversions and consequent increased space-time yields of a pre-esterification is disclosed, since, more particularly, the free fatty acids used for the increase in the proportion of the free fatty acids preferably stem from a later process step and are therefore subjected to further processing steps. This is especially true because soap cleavage by means of metered addition of homogeneously dissolved acids becomes necessary here. The maximum space-time yield disclosed in DE 196 00 025 is 34 g of fatty acid methyl ester per litre of reactor volume and hour.
For economically viable processing of fatty raw materials to biodiesel where the fatty raw materials contain a proportion of free fatty acids, it is therefore an object of the invention to develop a process which achieves esterification of the free fatty acids with high space-time yield and simultaneously enables recovery of the esterification catalyst.